Glossary of Weather Terms

Advection Fog - A fog that forms when warm air flows over a cold surface and cools from below until saturation is reached.

Aeroallergens - Any of a variety of allergens such as pollens, grasses, or dust carried by winds.

Air Mass - A body of air covering a relatively wide area and exhibiting horizontally uniform properties.

Amplitude - The maximum magnitude of a quantity. Often used to refer to the maximum height of a wave.

Anemometer - An instrument used for measuring the speed of the wind.

Anvil - The flat, spreading top of a cumulonimbus cloud, often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind.

Atmospheric Pressure - The pressure exerted by the earth's atmosphere at any given point, determined by taking the product of the gravitational acceleration at the point and the mass of the unit area column of air above the point.

Base Station - A focal point for data and communications. One base station can service one or more remote sites.

Barometric Pressure - The pressure of the atmosphere as indicated by a barometer.

Cold Front - A zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer.

Convective Clouds - The vertically developed family of clouds are cumulus and cumulonimbus. The height of their bases range from as low as 1,000 feet to a bit more than 10,000 feet. Clouds with extensive vertical development are positive indications of unstable air. Strong upward currents in vertically developed clouds can carry high concentrations of supercooled water to high levels where temperatures are quite cold. Upper portions of these clouds may be composed of water and ice.

Coriolis Force - A fictitious force used to account for the apparent deflection of a body in motion with respect to the earth, as seen by an observer on the earth. The deflection (to the right in the Northern Hemisphere) is caused by the rotation of the earth.

CRBasic - A datalogger program editor whose structure stems from the BASIC
programming language. CRBasic supports keyword, algebraic expressions, and conditional expressions. Dataloggers that use CRBasic include our CR200-series, CR800, CR1000, CR3000, CR5000, and CR9000(X).

Cumulus Buildups - Clouds which develop vertically due to unstable air. Characterized by their cauliflower-like or tower-like appearance of moderately large size

Cutoff Low - A closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression). "Cutoff low" and "closed low" often are used interchangeably to describe low pressure centers aloft. However, not all closed lows are completely removed from the influence of the basic westerlies. Therefore, the recommended usage of the terms is to reserve the use of "cutoff low" only to those closed lows which clearly are detached completely from the westerlies.

Datalogger - Also known as Measurement and Control System . Dataloggers are the brain
of a data acquisition system. They make measurements at a specified scan rate, process data, and initiate telecommunications. Our dataloggers also have control capabilities allowing them to automatically respond to a specified conditions by controlling external devices (e.g., opening flood gates, turning fans off/on).

Degree Day - A measure that gauges the amount of heating or cooling needed for a building using 65 degrees as a baseline. Electrical, natural gas, power, and heating, and air conditioning industries utilize heating and cooling degree information to calculate their needs. For more specific definitions and how to calculate degree days, see the definitions for Heating Degree Days and Cooling Degree Days.

Dew Point - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint).

Doppler Radar - Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna).

Eddy - Swirling currents of air at variance with the main current.

El Niño - A warming of the ocean current along the coasts of Peru and Ecuador that is generally associated with dramatic changes in the weather patterns of the region; a major El Niño event generally occurs every 3 to 7 years and is associated with changes in the weather patterns worldwide.

Evaporation - The process of a liquid changing into a vapor or gas, usually water in meteorology

Evapotranspiration - The amount of water no longer available in the soil due to evaporation and transpiration. Transpiration is the movement of liquid through a plant then released through the stomata of the plant as water vapor. There are several methods for estimating evapotranspiration including the Penman Monteith equation and Bowen ratio technique.

Freeze - A freeze is when the surface air temperature is expected to be 32°F or below over a widespread area for a climatologically significant period of time. Use of the term is usually restricted to advective situations or to occasions when wind or other conditions prevent frost. "Killing" may be used during the growing season when the temperature is expected to be low enough for a sufficient duration to kill all but the hardiest herbaceous crops.

Freezing Fog - A fog the droplets of which freeze upon contact with exposed objects and form a coating of rime and/or glaze.

Freezing Level - The altitude at which the air temperature first drops below freezing.

Freezing Rain - Rain that falls as a liquid but freezes into glaze upon contact with the ground.

Funnel Cloud - A condensation funnel extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado). A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.

Global Warming - An overall increase in world temperatures which may be caused by additional heat being trapped by greenhouse gases.

Gradient - A rate of change with respect to distance of a variable quantity, as temperature or pressure, in the direction of maximum change.

Growing Degree Day - The number of degrees that the average temperature is above a baseline value. For example, 40 degrees for canning purposes; 45 degree for potatoes; and 50 degrees for sweet corn, snap beans, lima beans, tomatoes, grapes, and field corn. Every degree that the average temperature is above the baseline value becomes a growing degree day. Agricultural related interests use growing degree days to determine planting times.

Hail - Showery precipitation in the form of irregular pellets or balls of ice more than 5 mm in diameter, falling from a cumulonimbus cloud.

High Clouds - These clouds have bases between 16,500 and 45,000 feet in the mid latitudes. At this level they are composed of primarily of ice crystals. Some clouds at this level are cirrus, cirrocumulus, and cirrostratus

Humidity - Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.

Hygrometer - A sensor that measures dew point.

Instability - The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.

Jet Stream - Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams). A jet stream at low levels is known as a low-level jet.

La Niña - La Niña, a phase of ENSO, is a periodic cooling of surface ocean waters in the eastern tropical Pacific along with a shift in convection in the western Pacific further west than the climatological average. These conditions affect weather patterns around the world. The preliminary CPC definition of La Niña is a phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal.

Land Breeze - A coastal breeze at night blowing from land to sea, caused by the difference in the rates of cooling of their respective surfaces.

Leeward - The side away from the wind. Compare windward.

Marine Inversion - Temperature inversion produced when cold marine air underlies warmer air.

Middle Clouds (or Mid-Level Clouds) - A term used to signify clouds with bases between 6,500 and 23,000 feet. At the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. Altocumulus, altostratus, and nimbostratus are the main types of middle clouds. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

Net Radiometer - A sensor that measures net flux of downward and upward total solar and terrestrial radiation through a horizontal surface.

Orographic Lifting - Same as Upslope Flow; occurs when air is forced to rise and cool due to terrain features such as hills or mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional cooling results in rain or snow. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain.

POPS - Probability of Precipitation

Precipitation - The process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc.

Pyranometer - A sensor that measures solar radiation.

Radiation Fog - A fog that forms when outgoing longwave radiation cools the near-surface air below its dew point temperature.

Radiational Inversion - Used interchangably with Nocturnal Inversion; a temperature inversion that develops during the night as a result of radiational cooling of the surface. Because the immediate surface (lower Boundary Layer) cools much more rapidly during these conditions than the air just above (upper Boundary Layer), a temperature inversion can be created overnight, but typically erodes quickly after sunrise.

Rain Gauge - An instrument for measuring the quantity of rain that has fallen.

Ridge - An elongated area of relatively high atmospheric pressure; the opposite of trough.

Santa Ana Wind - In southern California, a weather condition in which strong, hot, dust-bearing winds descend to the Pacific Coast around Los Angeles from inland desert regions.

Sea Breeze - A thermally produced wind blowing during the day from a cool ocean surface onto the adjoining warm land, caused by the difference in the rates of heating of the surfaces of the ocean and of the land.

Smoke Dispersal - Describes the ability of the atmosphere to ventilate smoke. Depends on the stability and winds in the lower layers of the atmosphere, i.e., a combination of mixing heights and transport winds.

Solar Radiation - Radiation originating from the sun.

Soil Moisture - Water contained in the upper part of the soil mantle. This moisture evaporates from the soil and is the used and transpired by vegetation.

Stable - An atmospheric state with warm air above cold air which inhibits the vertical movement of air.

Synoptic Scale - The spatial scale of the migratory high and low pressure systems of the lower troposphere, with wavelengths of 1000 to 2500 km.

Trough - An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

Transport Wind - The average wind over a specified period of time within a mixed layer near the surface of the earth.

Temperature Inversion (surface-based or elevated) - a layer of the atmosphere in which air temperature increases with height. When the layer's base is at the surface, the layer is called a surface-based temperature inversion; when the base of the layer is above the surface, the layer is called an elevated temperature inversion.

Tule Fog - Radiation fog in the Central Valley of California. It forms during night and morning hours in late fall and winter months following the first significant rainfall. A leading cause of weather related casualties in California.

Veering Winds - Winds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). The latter example is a form of directional shear which is important for tornado formation. Compare with backing winds.

Warm Advection - Transport of warm air into an area by horizontal winds. Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.

Wind Chill - Reference to the Wind Chill Factor; increased wind speeds accelerate heat loss from exposed skin, and the wind chill is a measure of this effect. No specific rules exist for determining when wind chill becomes dangerous. As a general rule, the threshold for potentially dangerous wind chill conditions is about -20°F

Windward - The side toward the wind. Compare with leeward.